The present invention relates to a hydrophone array and, in particular, to an array for use in seismic exploration and like applications.
Hydrophone arrays are used to perform active and passive sound imaging. One significant application of such arrays is in the field of geophysical surveying of seabeds. Hydrophone arrays generally utilise a plurality of acoustic transducers, spaced at a controlled spacing, for providing data which cannot be readily obtained from a single hydrophone.
According to a first aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of housings, each housing enclosing at least two acoustic transducers.
In a preferred embodiment, each transducer has an independent output and is able, in use, to sense the same tensor component of an elastic wave in the medium surrounding the hydrophone array.
The acoustic transducers act as acoustic receivers and acoustic transmitters as requirements dictate during use of the array. In one embodiment, the acoustic transducer can comprise a tube formed at least in part of piezoelectric material. The piezoelectric material can comprise an active polarised ceramic material, such as barium titanate or lead zirconate titanate (PZT). The piezoelectric material can, in another embodiment, be a piezoelectric polymer material, such as polyvinylidene fluoride (PVDF), or a piezo-rubber composite material.
The tube comprised at least in part of piezoelectric material can be substantially cylindrical. The tube can be fabricated from two segments or a greater number of segments. The tube can be sectioned longitudinally into longitudinal segments. The inner and outer faces of the segments can each have an electrically conductive material covering at least a part of each face and thereby provide an electrode for each face. Each electrode could comprise a coating of metal, such as silver, gold or aluminium or an intrinsically or extrinsically conductive polymer. Electrical connection to the faces can be facilitated by electrically conductive leads attached to the electrodes. The attachment of the leads to the electrodes can be through use of a conductive epoxy or adhesive metallic tape.
In the case where the tube is formed of a plurality of segments, the segments are preferably arranged such that electrical connection is provided between the respective inner faces of the segments and the respective outer faces of the segments. In one embodiment, the segments may be in an abutting arrangement with short electrical leads or conductive tape providing the electrical connection between the respective faces of the segments.
In a preferred embodiment of the invention, the piezoelectric polymer film can be bonded to a tubular support structure. In a still further embodiment, the support structure can comprise at least an inner and outer tube, with the inner tube mounted within the outer tube. The tubes preferably share a common longitudinal axis. An annular space is preferably located between the inner and outer tubes and extends at least a substantial length of the tubular structure. The inner surface of the outer tube is preferably bonded to the piezoelectric polymer film. The support structure can be fabricated from polycarbonate. The outer surface of the outer tube can also be coated with a metallic layer. This metallic layer can comprise a layer of metallic paint, such as spray paint, including nickel, gold or silver conductive paints. The outer metallic coat can have at least one electrically conductive wire connected thereto which is in turn connected to a shield or drain wire in the array.
In an alternative embodiment of the invention, the acoustic transducer can include one or more capacitors which are adapted to undergo a change in capacitance that is proportional to changes in incident pressure. In another embodiment, the acoustic transducer can comprise one or more resistors adapted to provide a varying resistance output in response to changes in incident pressure. In a still further embodiment, the acoustic transducer can comprise one or more inductors adapted to provide a varying inductance output in response to changes in incident pressure. In a still further embodiment, the acoustic transducer includes a combination of capacitors, resistors or inductors to provide an output that varies in response to changes in incident pressure. The varying outputs of the previously described acoustic transducers can be adapted to modulate the oscillation of a modulator and so provide a quantitative measurement of variations in incident pressure.
In a still further embodiment, the acoustic transducer can comprise a fiber optic transducer as is known in the art. Such hydrophones typically rely upon measuring changes in the behaviour of the light passed through a fiber optic guide due to acoustic waves being incident on the hydrophone. As an example only of one type of fiber optic hydrophone that can be used in the present invention, the fiber optic transducer can include an optical reflector that can undergo a displacement that is responsive to acoustic waves. A beam of light from a light source that is carried by a first group of fiber optic guides can be incident on the reflector. The light reflected from the reflector can be carried by a second group of fiber optic guides to a light detector. Any displacement of the reflector due to pressure waves impinging on the reflector are detected by changes in intensity of the reflected light from the light source.
In an alternative example of a fiber optic hydrophone, the hydrophone can include a first fiber optic acoustic wave detector which is subjected to incident acoustical waves. A similar fiber optic is also provided in the hydrophone in an acoustically isolated compartment where the optic fiber is not affected by the incident acoustical waves. A comparison of the light passing through each fiber optic can be used to determine the presence and magnitude of the incident acoustical wave on the hydrophone.
In a preferred embodiment, each hydrodynamic housing preferably encloses two acoustic transducers. A majority of the combined length of the two transducers is preferably positioned forward of the middle of each housing to minimise noise from turbulent boundary layer flow. The acoustic transducers are preferably arranged to be mounted around a cable. The transducers are preferably mounted symmetrically around the cable. The hydrophone array can be used as a towed, vertical or seismic downhole array. It is particularly suited to a towed sensor arrangement, where the cable is towed behind a geophysical surveying vessel. The cable could also be adapted to be laid on the sea-bed rather than towed behind a vessel in use as described above.
In another embodiment, each acoustic transducer can include:
a tube comprised at least in part of piezoelectric material and having an inner and outer surface;
a first shell member, having a first edge and a second edge, disposed outside the tube and extending longitudinally of the tube, the first and second edges being connected to or bearing against the outer surface of the tube; and
a second shell member, having a first and second edge disposed inside the tube and extending longitudinally of the tube, the first and second edges being connected to or bearing against the inside of the tube.
In this embodiment, the first and second shell members can extend the full longitudinal length of the tube and also preferably encase the outer and inner surfaces of the tube. Each transducer, in this embodiment, preferably has a shape which when viewed in perspective is substantially toroidal. The first and second shell members call be fabricated from a metal, such as an aluminium or bronze alloy to facilitate electrical connection. When each transducer, as described in this embodiment, is used as a receiver, the acoustic signal impinges as a pressure wave on the shell members causing the shell members to flex inwardly and outwardly in response to the signal. The inward and outward flexing of the shell members acts as a stress amplifier and leads to a corresponding amplification in the change in the longitudinal extension of the piezoelectric tube which in turn produces a corresponding electric signal.
In the case of an array that will be towed by a vessel, the cable in such an application must provide towing forces for the acoustic transducers and desirably the electrical and other signal connections necessary between each acoustic transducer and the associated data processing and analysis equipment which is normally located in the vessel towing the array. Signal transmission through the cable may be through standard bearers or through the use of optical fibers.
When mounted on a cable, a strain shielding element is preferably provided on the cable for each acoustic transducer. The shielding element serves to shield the transducer from stresses generated in the cable. The shielding element preferably consists in a cylindrical member that fits snugly around the cable. In one embodiment, the shielding element can be sectioned longitudinally into longitudinal segments. The cylindrical member is preferably fabricated from a stiff material such as a metal, alloy or carbon fiber.
The acoustic transducers within each housing are separated by a short distance. This distance can be filled with a relatively soft material, such as a polyurethane composite. In the volume extending forwardly from the front acoustic transducer, the housing can be substantially filled with a relatively hard syntactic composite material. In the volume extending rearwardly from the rear-most transducer, the housing can be filled with a relatively soft syntactic composite material. The syntactic composite materials in the housing serve to provide the housing with a degree of buoyancy which is particularly useful when the array is being used as a towed array.
In one embodiment, each housing is preferably hydrodynamically smoothly shaped. Each housing in this embodiment can have a nose section comprising a smoothly increasing cross-sectional diameter from a point of attachment to the cable to a point of maximum cross-sectional diameter. The housing can also smoothly decrease in cross-sectional diameter from the point of maximum cross-sectional diameter to a point of attachment to a tail bridle that defines a tail section of the housing. The point of maximum cross-section diameter is preferably located in this embodiment closer to the tail bridle attachment than to the point of attachment to the towing cable. The housing in this embodiment can be circular in cross-sectional diameter. Further, the housing, when viewed in length-wise cross-section, can define a curve that is everywhere continuously second order differential. The housing can be formed from a material having varying flexural rigidity, with the housing being more flexible in the tail section thereof than in the nose section thereof. Where the array comprises a plurality of housings as described in this paragraph, the distance between the respective housings can be generally equal to one half wave length at the highest frequency of interest. Alternatively, the housings may be randomly or logarithmically spaced such that the nearest distance between two housings is greater than one housing length.
In another embodiment, the cable and plurality of housings can be enclosed within an outer sheath of substantially constant external dimensions which extends substantially the full length of the cable. The sheath can be comprised of a plastics material and provide a water-tight enclosure for the cable and housings. The sheath can be formed from a relatively soft and relatively smooth material, such as thermoplastic rubber or polyurethane. In another embodiment, the sheath can be formed from a non-watertight material. The sheath can be filled with a gel, fluid, or plastics material. In this embodiment, the sheath can be cylindrical or elliptical in cross-section.
In a further embodiment, the housing can have a first bend limiting means extending forwardly from its nose section. The housing in a still further embodiment can have a second bend limiting means extending rearwardly from its tail section. In one embodiment, the first and/or second bend limiting means can at least partially overlap with the housing. Where there is an overlap with the housing, the thickness of the bend limiting means can also decrease along the housing. The first bend limiting means can taper in thickness away from the nose section. The second bend limiting means can also taper in thickness away from the tail section. The first and second bend limiting means are respectively adapted to minimise the degree of bending of the array proximate the housing. While some bending of the array can occur while the array is deployed, tight bending of the cable can lead to damage to the cable and housing when the cable is in a rolled configuration. By minimising the degree of curvature, the bend limiting means reduce the likelihood of the array being rolled up to a degree that damages the array.
The first and second bend limiting means can be formed from a polyurethane or polyurethane composite material. The respective bend limiting means can have a Shore A 60-70 hardness.
The towing cable is preferably comprised of a plurality of aramid fibres. The aramid fibres can be coated with a paraffin wax compound or similar to prevent water ingress into the aramid fibres. The towing cable is preferably surrounded by a plurality of data bearers that extend between each of the housings and the associated data processing and analysis equipment. The data bearers can be jacketed with a plastics material, such as polyethylene. Such a lightweight, low density and slippery plastics material jacket serves to lower the density of the entire array and allows the data bearers to withstand a greater number of bending cycles.
The hydrophone array can include at least one velocity sensor and/or accelerometer. These sensors can be mounted in each housing of the array. The sensors can be mounted, in one embodiment, between the inner surface of the support structure and the outer surface of the strain shielding element. The sensors are preferably adapted to allow determination and monitoring of the different components of motion of the housings included in the array.
The signals generated by the at least two acoustic transducers in the array are preferably amplified in a preamplifier circuit. Such a transducer/preamplifier combination can include a calibration signal input that injects a calibration signal through the transducers, preamplifiers, and other signal conditioning electronics permitting amplitude and phase differences between channels to be removed in later digital or analog processing. In a preferred embodiment, the calibration signal is injected differentially into a differential pair of transducers. Such a result may be achieved by putting a resistor between each piezoelectric transducer of a differential pair and ground, and injecting the calibration signal at the resistor-transducer junctions. The signals can also be passed through signal conditioning electronics. Such signal conditioning electronics can include multiple pole low cut filtering before any digitisation process is applied to preserve maximum dynamic range in the presence of very low frequency noise sources such as ocean swells.
According to a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of hydrodynamically shaped housings, each housing enclosing at least one acoustic transducer, said transducer comprising one or more capacitors which undergo a change in capacitance that is proportional to changes in incident pressure on said housing.
According to yet a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of hydrodynamically shaped housings, each housing enclosing at least one acoustic transducer, said acoustic transducer comprising one or more resistors which provide a varying resistance output in response to changes in incident pressure on said housing.
According to a still further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of hydrodynamically shaped housings, each housing enclosing at least one acoustic transducer, said acoustic transducer comprising one or more inductors which provide a varying inductance output in response to changes in incident pressure on said housing.
According to a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of hydrodynamically shaped housings, each housing enclosing at least one acoustic transducer, said acoustic transducer comprising a combination of capacitors, resistors or inductors which provide an output that varies in response to changes in incident pressure on said housing.
According to yet a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of hydrodynamically shaped housings, each housing enclosing at least one acoustic transducer, said acoustic transducers comprising a fiber optic transducer.
According to a still further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of housings, the housings being enclosed within an outer sheath of substantially constant external dimensions which extends substantially the full length of the array, each housing enclosing at least one acoustic transducer, said transducer comprising one or more capacitors which undergo a change in capacitance that is proportional to changes in incident pressure on said housing.
According to a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of housings, the housings being enclosed within an outer sheath of substantially constant external dimensions which extends substantially the full length of the array, each housing enclosing at least one acoustic transducer, said acoustic transducer comprising one or more resistors which provide a varying resistance output in response to changes in incident pressure on said housing.
According to yet a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of housings, the housings being enclosed within an outer sheath of substantially constant external dimensions which extends substantially the full length of the array, each housing enclosing at least one acoustic transducer, said acoustic transducer comprising one or more inductors which provide a varying inductance output in response to changes in incident pressure on said housing.
According to a still further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of housings, the housings being enclosed within an outer sheath of substantially constant external dimensions which extends substantially the full length of the array, each housing enclosing at least one acoustic transducer, said acoustic transducer comprising a combination of capacitors, resistors or inductors which provide an output that varies in response to changes in incident pressure on said housing.
According to a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of housings, the housings being enclosed within an outer sheath of substantially constant external dimensions which extends substantially the full length of the array, each housing enclosing at least one acoustic transducer, said acoustic transducers comprising a fiber optic transducer.
According to yet a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of hydrodynamically shaped housings, each housing enclosing at least one acoustic transducer, the housing having a volume between the cable and the transducer that is substantially filled with syntactic composite material.
According to a still further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of hydrodynamically shaped housings, each housing enclosing at least one acoustic transducer, the housing having a volume extending forwardly from the forwardmost of said at least one acoustic transducer in said housing, said volume being substantially filled with a syntactic composite material.
According to a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of hydrodynamically shaped housings, each housing enclosing at least one acoustic transducer, the housing having a volume extending rearwardly from the rearmost of said at least one acoustic transducer in said housing, said volume being filled with a syntactic composite material.
According to yet a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of hydrodynamically shaped housings, each housing enclosing at least one acoustic transducer, the housing having a first volume extending forwardly from the forwardmost of said at least one acoustic transducer in said housing, said first volume being substantially filled with a relatively hard syntactic composite material, and a second volume extending rearwardly from the rearmost of said at least one acoustic transducer in the housing, said second volume being filled with a relatively soft syntactic composite material.
According to a still further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of housings, the housings being enclosed within an outer sheath of substantially constant external dimensions which extends substantially the full length of the array, each housing enclosing at least one acoustic transducer, the housing having a volume between the cable and the transducer that is substantially filled with syntactic composite material.
According to a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of housings, the housings being enclosed within an outer sheath of substantially constant external dimensions which extends substantially the full length of the array, each housing enclosing at least one acoustic transducer, the housing having a volume extending forwardly from the forwardmost of said at least one acoustic transducer in said housing, said volume being substantially filled with a syntactic composite material.
According to yet a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of housings, the housings being enclosed within an outer sheath of substantially constant external dimensions which extends substantially the full length of the array, each housing enclosing at least one acoustic transducer, the housing having a volume extending rearwardly from the rearmost of said at least one acoustic transducer in the housing, said volume being filled with a syntactic composite material.
According to a still further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of housings, the housings being enclosed within an outer sheath of substantially constant external dimensions which extends substantially the full length of the array, each housing enclosing at least one acoustic transducer, the housing having a first volume extending forwardly from the forwardmost of said at least one acoustic transducer in said housing, said first volume being substantially filled with a relatively hard syntactic composite material, and a second volume extending rearwardly from the rearmost of said at least one acoustic transducer in the housing, said second volume being filled with a relatively soft syntactic composite material.
According to a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of hydrodynamically shaped housings, each housing enclosing at least one acoustic transducer and having a first bend limiting means extending forwardly from a nose section thereof and a second bend limiting means extending rearwardly from a tail section thereof.
In this aspect, one or both of the first and second bend limiting means can at least partially overlap with the housing. Further, the first bend limiting means can taper in thickness away from said nose section and said second bend limiting means can taper in thickness away from said tail section.
According to yet a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of hydrodynamically shaped housings, each housing enclosing at least one acoustic transducer and at least one velocity sensor.
According to a still further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of hydrodynamically shaped housings, each housing enclosing at least one acoustic transducer and at least one accelerometer sensor.
According to a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of housings, the housings being enclosed within an outer sheath of substantially constant external dimensions which extends substantially the full length of the array, each housing enclosing at least one acoustic transducer and at least one velocity sensor.
According to yet a further aspect, the present invention comprises a hydrophone array including a cable having attached thereto a plurality of housings, the housings being enclosed within an outer sheath of substantially constant external dimensions which extends substantially the full length of the array, each housing enclosing at least one acoustic transducer and at least one accelerometer sensor.